U.S. patent application number 13/018109 was filed with the patent office on 2012-04-12 for direct laser modulation.
Invention is credited to Pietro Arturo Bernasconi, David Thomas Neilson.
Application Number | 20120087655 13/018109 |
Document ID | / |
Family ID | 46051315 |
Filed Date | 2012-04-12 |
United States Patent
Application |
20120087655 |
Kind Code |
A1 |
Neilson; David Thomas ; et
al. |
April 12, 2012 |
Direct Laser Modulation
Abstract
An apparatus includes an array of lasers, an array of electrical
drivers, and optical filter. Each laser is configured to produce
light in a corresponding wavelength-channel, wherein the
wavelength-channels of different ones of the lasers are different.
The electrical drivers are connected to directly modulate the
lasers. Each driver produces a first driving current or voltage to
cause a corresponding one of the lasers to be in a first lasing
state and produces a different second driving current or voltage to
cause the corresponding one of the lasers to be in a different
second lasing state. The optical filter is connected to receive
light output by the lasers. The optical filter selectively
attenuates light from each of the lasers in the first lasing states
thereof and to selectively pass light from each of the lasers in
second lasing states thereof.
Inventors: |
Neilson; David Thomas; (Old
Bridge, NJ) ; Bernasconi; Pietro Arturo; (Aberdeen,
NJ) |
Family ID: |
46051315 |
Appl. No.: |
13/018109 |
Filed: |
January 31, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12945429 |
Nov 12, 2010 |
|
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13018109 |
|
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61390876 |
Oct 7, 2010 |
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Current U.S.
Class: |
398/34 ;
372/29.015; 398/91 |
Current CPC
Class: |
G02F 1/23 20130101; G02B
5/20 20130101; H04B 10/506 20130101; H04J 14/0221 20130101; H01S
5/0427 20130101 |
Class at
Publication: |
398/34 ;
372/29.015; 398/91 |
International
Class: |
H04J 14/02 20060101
H04J014/02; H04B 10/08 20060101 H04B010/08; H01S 3/10 20060101
H01S003/10 |
Claims
1. An apparatus comprising: an array of lasers, each of the lasers
of the array being configured to produce light in a corresponding
wavelength-channel, the wavelength-channels of different ones of
the lasers being different; an array of electrical drivers
connected to directly modulate the lasers, each driver being
configured to produce a first driving current or voltage to cause a
corresponding one of the lasers to be in a first lasing state and
to produce a different second driving current or voltage to cause
the corresponding one of the lasers to be in a different second
lasing state; and an optical filter connected to receive light
output by the lasers of the array; and wherein the optical filter
is configured to selectively attenuate light from each of the
lasers in the first lasing states thereof and to selectively pass
light from the each of the lasers in the second lasing states
thereof.
2. The apparatus of claim 1, wherein the wavelength-channels of the
lasers have an average spacing, and the average wavelength-channel
spacing is about equal to a positive integer multiple of a free
spectral range of the optical filter.
3. The apparatus of claim 1, wherein the wavelength-channels of the
lasers are tunable to lie on a fixed grid, the grid having an
average wavelength-channel spacing that is about equal to a
positive integer multiple of a free spectral range of the optical
filter.
4. The apparatus of claim 2, wherein the average wavelength-channel
spacing is equal to the free spectral range of the optical filter
.+-.10 percent or less of the average wavelength-channel
spacing.
5. The apparatus of claim 1, wherein the average spacing is equal
the free spectral range of the optical filter .+-.10 or less of the
average wavelength-channel spacing.
6. The apparatus of claim 1, wherein each laser outputs light
having a first center wavelength in response to being directly
modulated with a first digital data value and outputs light having
a different second center wavelength in response to being directly
modulated with a different second digital data value and the
optical filter has a response that is at least 2 decibels smaller
at each second center wavelength than at the first center
wavelength of the same laser.
7. The apparatus of claim 6 wherein the wavelength-channels of the
lasers have an average wavelength-channel spacing, and the average
wavelength-channel spacing is about equal to a positive integer
multiple of a free spectral range of the optical filter.
8. The apparatus of claim 1, further comprising: a first
photo-detector configured to receive light being output by the
lasers and not having passed through the optical filter; and a
second photo-detector configured to receive light being output by
the lasers and having passed through the optical filter.
9. The apparatus of claim 8, further comprising a controller
configured to adjust output wavelengths of the lasers based on
light intensity measurements of the first and second
photo-detectors.
10. The apparatus of claim 8, wherein the apparatus is configured
to dither each of the lasers.
11. The apparatus of claim 9, wherein the controller is configured
to estimate relative contributions of individual ones of the lasers
to the intensity of light measured by the second
photo-detector.
12. The apparatus of claim 9, wherein the optical filter has a free
spectral range that is equal to the average wavelength-channel
spacing for light output by the array of the lasers .+-.10 percent
or less of the average wavelength-channel spacing.
13. The apparatus of claim 11, wherein the controller is configured
to adjust an output wavelength a particular one of the lasers by
estimating a light intensity detected by the second photo-detector
at a frequency of the dithering of the particular one of the
lasers.
14. A method, comprising: directly modulating a plurality of lasers
such that each one of the lasers produces an optical carrier that
is modulated to carry digital data in a wavelength-channel
corresponding to the one of the lasers, the wavelength-channels
being spaced apart by an average wavelength-channel spacing;
forming a multiplexing light beam by optically multiplexing the
modulated optical carriers; and optically filtering a first portion
of the multiplexed light beam with an optical filter having a free
spectral range, the average fixed wavelength-channel spacing being
about equal to a positive integer multiple of the free spectral
range.
15. The method of claim 14, wherein each directly modulated laser
lases with a corresponding first center wavelength when
transmitting digital data of a first value and lases with a
corresponding different second center wavelength when transmitting
digital data of a different second value, the filtering attenuating
light at the second center wavelength by at least 2 decibels more
than light at the first center wavelength.
16. The method of claim 14, further comprising: splitting the
multiplexed light beam in a substantially wavelength-independent
manner to form the first portion of the multiplexed light beam and
a first part of the multiplexed light beam; measuring an intensity
of the first part of the multiplexed light without passing the
first part through the optical filter; and measuring an intensity
of a second part of the filtered first portion of the multiplexed
light, the second part being produced by splitting the filtered
first portion into the second part and another portion of the
filtered portion in a substantially wavelength-independent
manner.
17. The method of claim 16, further comprising adjusting output
wavelengths of the lasers based on one or more of the measured
intensities.
18. The method of claim 17, further comprising dithering output
wavelengths of the lasers during the measuring acts.
19. The method of claim 18, wherein the adjusting the output
wavelength of each individual one of the lasers includes estimating
a contribution to the measured intensity of the second part at
dithering frequency of the individual one of the lasers.
20. The method of claim 17, wherein each directly modulated laser
lases with a corresponding first center wavelength when
transmitting digital data of a first value and lases with a
corresponding second center wavelength when transmitting digital
data of a different second value, the filtering attenuating light
at the second center wavelength by at least 2 decibels more than
light at the first center wavelength.
21. The method of claim 16, wherein a free spectral range of the
optical filter is equal to the average wavelength-channel spacing
.+-.10 percent or less of the average wavelength-channel spacing.
Description
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 12/945,429, which was filed on Nov. 12, 2010,
and also claims the benefit of U.S. provisional applications
61/390,876; 61/390,837; 61/390,840; and 61/390,798, which were all
filed on Oct. 7, 2010.
BACKGROUND
[0002] 1. Technical Field
[0003] The inventions relate to apparatus and methods fig optical
communication.
[0004] 2. Discussion of the Related Art
[0005] This section introduces aspects that may be helpful to
facilitating a better understanding of the inventions. Accordingly,
the statements of this section are to be read in this light and are
not to be understood as admissions about what is in the prior art
or what is not in the prior art.
[0006] In optical communication systems, a data stream is modulated
onto an optical carrier, which carries the data from an optical
transmitter to an optical receiver. One common method of optical
data modulation involves amplitude modulation of the optical
carrier, e.g., between ON and OFF states. Such optical amplitude
modulation of the optical carrier may be performed by direct
modulation of the laser that produces the optical carrier, e.g., by
changing the biasing of the laser whose output is being modulated.
Such optical amplitude modulation of the optical carrier may
alternatively be performed by externally modulating the light beam
output by a laser, e.g., with a variable optical attenuator.
BRIEF SUMMARY
[0007] Various embodiments provide for optically communicating data
based on an array of directly modulated lasers. Some such
embodiments provide apparatus and/or methods for wavelength-locking
an array of directly modulated lasers that produce the
data-modulated optical carriers for an optical communications
system.
[0008] One embodiment of an apparatus includes an array of lasers,
an array of electrical drivers, and an optical filter. Each laser
is configured to produce light in a corresponding
wavelength-channel, wherein the wavelength-channels of different
ones of the lasers are different. The electrical drivers are
connected to directly modulate the lasers. Each driver produces a
first driving current or voltage to cause a corresponding one of
the lasers to be in a first lasing state and produces a different
second driving current or voltage to cause the corresponding one of
the lasers to be in a different second lasing state. The optical
filter is connected to receive light output by the lasers. The
optical filter selectively attenuates light from each of the lasers
in the first lasing states thereof and to selectively passes light
from each of the lasers in the second lasing states thereof.
[0009] In some embodiments of the above apparatus, the
wavelength-channels of the lasers optical filter may have an
average wavelength-channel spacing, and the average
wavelength-channel spacing is about equal to a positive integer
multiple of a free spectral range of the optical filter. The
average wavelength-channel spacing may be equal, e.g., to a
positive integer multiple of the free spectral range .+-.10 percent
or less of the average wavelength-channel spacing.
[0010] In some embodiments of any of the above apparatus, each
laser may output light having a first center wavelength in response
to being in directly modulated with a first digital data value and
may output light having a different second center wavelength in
response to being directly modulated with a different second
digital data value. In such embodiments, the optical filter may
have a response that is at least 2 decibels smaller at each second
center wavelength than at the first center wavelength of the same
laser.
[0011] In some embodiments of any of the above apparatus, the
apparatus may further include first and second photo-detectors. The
first photo-detector is configured to receive light being output by
the lasers and not having passed through the optical filter. The
second photo-detector is configured to receive light being output
by the lasers and having passed through the optical filter. In some
such embodiments, the apparatus may also include a controller
configured to adjust output wavelengths of the lasers based on
light intensity measurements of the first and second
photo-detectors.
[0012] In some embodiments of any of the above apparatus, the
apparatus may be configured to dither each of the lasers. In some
such embodiments, the controller may be configured to adjust an
output wavelength of a particular one of the lasers by estimating a
light intensity detected by the second photo-detector at a
frequency of the dithering of the particular one of the lasers.
[0013] In some embodiments of any of the above apparatus, the
apparatus may be configured to modulate a corresponding
pseudo-random sequence with each of the lasers. In some such
embodiments, the controller may also be configured to adjust an
output wavelength of a particular one of the lasers by estimating
that part of a light intensity detected by the second
photo-detector that correlates with the pseudo-random sequence
corresponding to the particular one of the lasers.
[0014] In some embodiments of any of the above apparatus, the
controller may be configured to estimate relative contributions of
individual ones of the lasers to the intensity of light measured by
the second photo-detector.
[0015] In some embodiments of any of the above apparatus, the
optical filter may have a free spectral range that is approximately
equal to the average wavelength-channel spacing for light output by
the array of the lasers .+-.10 percent or less of the average
wavelength-channel spacing.
[0016] In some embodiments, a method includes directly modulating a
plurality of lasers such that each one of the lasers produces an
optical carrier that is modulated to carry digital data in a
wavelength-channel corresponding to the one of the lasers. The
method also includes forming a multiplexing light beam by optically
multiplexing the modulated optical carriers and optically filtering
a first portion of the multiplexed light beam with an optical
filter having a free spectral range. The wavelength-channels are
spaced apart by an average wavelength-channel spacing that is about
equal to a positive integer multiple of the free spectral
range.
[0017] In some embodiments of the above method, each of the
directly modulated lasers may lase with a corresponding first
center wavelength when transmitting digital data of a first value
and lase with a corresponding different second center wavelength
when transmitting digital data of a different second value. The
optical filtering may attenuate light at the second center
wavelength by at least 2 decibels more than light at the first
center wavelength.
[0018] In some embodiments of any of the above methods, the methods
further include splitting the multiplexed light beam in a
substantially wavelength-independent manner to form the first
portion of the multiplexed light beam and a first part of the
multiplexed light beam. In such embodiments, the methods also
include measuring an intensity of the first part of multiplexed
light without passing the first part through the optical filter and
measuring an intensity of a second part of the filtered first
portion of the multiplexed light. The second part is produced by
splitting the filtered first portion into the second part and
another portion of the filtered portion in a substantially
wavelength-independent manner. In any such embodiments, the methods
may further include adjusting output wavelengths of the lasers
based on one or more of the measured intensities. In any such
embodiments, the methods may further include dithering output
wavelengths of the lasers during the measuring acts. In any such
embodiments, the adjusting the output wavelength of each individual
one of the lasers may include estimating a contribution to the
measured intensity of the second part at the dithering frequency of
the individual one of the lasers.
[0019] In some embodiments of any of above methods, each directly
modulated laser lases with a corresponding first center wavelength
when transmitting digital data of a first value and lases with a
corresponding second center wavelength when transmitting digital
data of a different second value. In such embodiments, the
filtering attenuates light at the second center wavelength by at
least 2 decibels more than light at the first center
wavelength.
[0020] In any of the above embodiments of methods, the free
spectral range of the optical filter may equal the average
wavelength-channel spacing .+-.10 percent or less of the average
wavelength-channel spacing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A, 1B, and 1C schematically illustrate first second
and third examples of optical communication systems that directly
modulate one or more lasers and optically filter the modulated
optical carrier(s) in an optical transmitter, an optical receiver,
and an optical channel, respectively.
[0022] FIG. 2 schematically illustrates an example of an output
spectrum of a laser during direct laser modulation and an example
of a filter response that might be used to optically filter the
output spectrum in an optical communication system or an optical
subsystem, e.g., the optical systems of FIGS. 1A-1C and the optical
subsystem of FIG. 3.
[0023] FIG. 3 schematically illustrates an optical subsystem with
an optical filter, e.g., used to improve amplitude optical
extinction ratio(s) and/or perform wavelength-channel locking,
e.g., in any of the optical communication systems of FIGS.
1A-1C.
[0024] FIG. 4 is a flow chart illustrating a method of processing
light output by one or more directly modulated laser(s) of an
array, e.g., to improve amplitude optical extinction ratio(s),
e.g., in the systems and subsystems of any of FIGS. 1A, 1B, 1C, and
3.
[0025] FIG. 5 is a flow chart illustrating a method of adjusting
the output wavelengths of any array of directly modulated lasers,
e.g., in the systems and subsystems of any of FIGS. 1A, 1B, 1C, and
3, e.g., to implement wavelength-locking.
[0026] FIGS. 6A-6C schematically illustrate how the intensity of an
amplitude and/or frequency and/or frequency modulated optical
carrier from a directly modulated laser may change due to filtering
with an optical filters having different alignments with respect to
the major spectral peak in the laser's output spectrum, e.g., in
the systems and subsystem of any of FIGS. 1A, 1B, 1C and 3.
[0027] In the Figures and text, like reference symbols indicate
elements with similar or the same function and/or structure.
[0028] In the Figures, relative dimension(s) of some feature(s) may
be exaggerated to more clearly illustrate the feature(s) and/or
relation(s) to other feature(s) therein.
[0029] Herein, various embodiments are described more fully by the
Figures and the Detailed Description of Illustrative Embodiments.
Nevertheless, the inventions may be embodied in various forms and
are not limited to the embodiments described in the Figures and the
Detailed Description of Illustrative Embodiments.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0030] U.S. provisional applications 61/390876, 61/390837,
61/390840, and 61/390798, which were all filed on Oct. 7, 2010;
U.S. application Ser. No. 12/944,939, "OPTICAL ASSEMBLY FOR A WDM
RECEIVER OR TRANSMITTER", which was filed on Nov. 12, 2010, by
David T. Neilson, Nagesh R. Basavanhally, and Mark Earnshaw (Docket
No. 807934-US-NP); U.S. application Ser. 12/944,875,
"OPTO-ELECTRONIC ASSEMBLY FOR A LINE CARD", which was filed on Nov.
12, 2010, by Mark Earnshaw (Docket No. 807933-US-NP); U.S.
application Ser. 12/944,917, "OPTICAL TRANSMITTER WITH FLIP-CHIP
MOUNTED LASER OR INTEGRATED ARRAYED WAVEGUIDE GRATING WAVELENTH
DIVISION MULTIPLEXER", which was filed on Nov. 12, 2010, by Mark
Earnshaw and Flavio Pardo (Docket No. 807931-US-NP); U.S.
application Ser. No. 12/944,946, "THERMALLY CONTROLLED
SEMICONDUCTOR OPTICAL WAVEGUIDE", which was filed on Nov. 12, 2010,
by Mahmoud Rasras (Docket No. 808553-US-NP); and U.S. application
Ser. No. 12/945,550 "WAVELENGTH ALIGNING MULTI-CHANNEL OPTICAL
TRANSMITTERS", which was filed on Nov. 12, 2010, by Douglas M. Gill
(Docket No. 808555-US-NP), are all incorporated herein by reference
in their entirety. One or more of the above applications may
describe optical transmitter structures and/or optical receiver
structures; methods of making optical receiver structures and/or
optical transmitter structures; and/or methods of using optical
receivers, optical transmitters, and components thereof that may be
suitable for use in or with, making of, and/or use of embodiments
and/or components of embodiments as described herein.
[0031] Various embodiments may provide for optical
wavelength-locking of or more directly modulated lasers of an array
and/or may provide for improving amplitude optical extinction
ratio(s) in one or more directly modulated lasers of the array.
[0032] FIGS. 1A-1C schematically illustrate alternate embodiments
10A-10C of optical communication systems 10A that include an
optical transmitter 2, an optical transmission channel 4, and an
optical receiver 6. The optical transmitter 2 directly modulates
digital data onto an optical carrier and transmits the modulated
optical carrier to the optical receiver 6 via the optical
transmission channel 4. The optical transmission channel 4 may be
an optical waveguide; an optical fiber line, an all-optical
single-span or multi-span or a non-all optical multi-span line of
optical transmission fibers; or a free-space optical channel. The
optical receiver 6 processes the received modulated optical carrier
and extracts digital data modulated thereon.
[0033] Each optical communication system 10A-10C includes an
optical communications subsystem that has an array of N laser(s)
12.sub.1, . . . , 12.sub.N, an array of N corresponding laser
driver(s) 14.sub.1, . . . , 14.sub.N, an optical filter 18 and
optionally an N.times.1 optical multiplexer or power combiner 16.
Herein, N refers to a positive integer, e.g., N may be 1, 2, 3, 4,
5, 6, 7, 8, 9, 10 or a larger integer.
[0034] Each of the N lasers 12.sub.1, . . . , 12.sub.N can output
an amplitude and/or frequency modulated optical carrier during
direct laser modulation, i.e., light beams as schematically
illustrated by arrowed lines in FIGS. 1A-1C. The light of each such
optical carrier is typically in a separate wavelength-channel. The
wavelength-channels form a sequence of adjacent channels that have
an average wavelength-channel spacing (AWCS). The average
wavelength-channel spacing is the actual wavelength-channel spacing
if the spacing is wavelength-independent, but otherwise differs
from the actual wavelength-channel spacing. Each amplitude and/or
frequency modulated optical carrier may carry a sequential data
modulation corresponding to a digital data sequence received by the
optical transmitter e.g., data sequences DATA.sub.1-DATA.sub.N in
FIGS. 1A-1C.
[0035] Each laser driver 14.sub.1, . . . , 14.sub.N electrically
connects to and controls a corresponding laser 12.sub.1, . . . ,
12.sub.N of the array. Each laser driver 14.sub.j can transform the
corresponding laser 12.sub.j between a fixed set of lasing states
responsive to digital data received by that laser driver 14.sub.j.
The laser driver 14.sub.j may transform the lasing states by
adjusting a biasing current or voltage or by adjusting a pumping
current of the corresponding laser 12.sub.j. Each fixed set of
lasing states includes two or more states, and the laser's output
intensity or wavelength or both differ(s) in the different states
of the fixed set. Typically, the output intensity, the wavelength,
or both differ in the different states of the fixed set as
discussed below. Thus, each laser driver 14.sub.j directly
modulates the corresponding laser 12.sub.j to output an optical
carrier that is amplitude and/or frequency modulated to carry the
digital data sequence DATA.sub.1 received by that laser driver
14.sub.j.
[0036] The optional N.times.1 optical coupler 16 may connect to the
optical outputs of the N lasers 12.sub.1-12.sub.N and deliver light
from the lasers 12.sub.1-12.sub.N to the optical input of the
optical filter 18 as a optical wavelength-multiplexed light beam.
The N.times.1 optical multiplexer 16 may be, e.g., a conventional
arrayed waveguide optical multiplexer or an optical diffraction
grating or a set of thin film optical filters such as dichroic
films. The light from the N lasers 12.sub.1-12.sub.N may
alternatively be directed separately to the optical input of the
optical filter 18, e.g., via bulk optical device(s) (not
shown).
[0037] The optical filter 18 receives the amplitude and/or
frequency modulated light beams from the lasers 12.sub.1-12.sub.N.
The optical filter 18 may be in the optical transmitter 4, the
optical receiver 6, or the optical transmission channel 4 as
illustrated in FIGS. 1A, 1B, and 1C, respectively.
[0038] In some embodiments, the optical filter 18 improves
amplitude optical extinction ratio(s) of some or all of the
modulated light beams output the lasers 12.sub.1-12.sub.N of the
array, i.e., so that the modulated light beams have an improved
amplitude modulation thereon. Indeed, the lasers 12.sub.1-12.sub.N
may produce modulated light beams with undesirably low amplitude
optical extinction ratios, e.g., 3-5 decibels (dB), because their
amplitude modulation has a low depth during direct laser
modulation. The depth of the amplitude modulation maybe kept low in
various embodiments so that the lasers 12.sub.1-12.sub.N do not
have operating instabilities when directly modulated at high speeds
where a temporal response lag might occur in carrier currents
therein. But, a low modulation depth usually implies that a laser
will produce a modulated optical carrier whose amplitude extinction
ratio is low, which means that the amplitude modulation of the
optical carrier has a large DC optical component. Such a large DC
optical component can substantially and undesirably lower
signal-to-noise ratios at an optical receiver that demodulates data
from a received optical carrier based on its amplitude modulation.
Such a large DC optical component or equivalently a low amplitude
optical extinction ratio could be particularly problematic when the
optical receiver is distant: from the optical transmitter
transmitting the modulated optical carrier. Thus, in some
embodiments of the optical communication systems 10A-10C, it may be
desirable to improve amplitude optical extinction ratios of the
amplitude and/or frequency modulated optical carriers output by
some or all of the N lasers 12.sub.1-12.sub.N.
[0039] In addition, some embodiments may be configured to directly
modulate some or all the N lasers 12.sub.1-12.sub.N output
frequency modulated optical carriers with low or minimal amplitude
modulation. In such embodiments, optical filtering in the optical
filter 18 may introduce an amplitude modulation with a suitable
amplitude extinction ratio unto such frequency modulated optical
carriers.
[0040] FIG. 2 schematically illustrates the action of one
embodiment of the optical filter 18 of FIGS. 1A-1C. This embodiment
of the optical filter 18 may be used to improve amplitude optical
extinction ratio(s) in one or more of the amplitude and/or
frequency modulated optical carriers output by the lasers
12.sub.1-12.sub.N of FIGS. 1A-1C and/or may be used to optically
wavelength-lock one or more of the N lasers 12.sub.1-12.sub.N to
corresponding predetermined output wavelength channel(s).
[0041] FIG. 2 schematically illustrates output spectra of the j-th
and (j+1)-th wavelength-channels (solid lines) of two lasers with
nearest neighboring wavelength-channels, and the filter response
(dashed lines) of the optical filter 18.
[0042] In this embodiment, each laser 12.sub.1-12.sub.N switches
between two laser states during direct laser modulation, i.e., a
high output intensity state H and a low output intensity state L.
In these two states, the laser's optical cavity has slightly
different optical path lengths. The different optical path lengths
may be caused, e.g., by different carrier densities in the laser's
optical cavity and/or may be caused, e.g., by the cavity's
electro-optical response to different laser driving voltages. Due
to the different optical path lengths, each laser 12.sub.1-12.sub.N
produces light in an H spectral peak while operating in the H
state, and outputs light in an L spectral peak of slightly
different wavelength while operating in the L state.
[0043] As illustrated in FIG. 2, the embodiment of the optical
filter 18 has a wavelength-dependent response for which some pass
bands are approximately centered on the H peak(s) of the lasers
12.sub.1-12.sub.N. These band passes of the optical filter 18
substantially pass light of the corresponding H peak and strongly
attenuate light of the peak L for the same laser 12.sub.1-12.sub.N,
because an edge of each of such band pass is located between the
corresponding pair of H and L peaks or close to the corresponding
pair of H and L peaks. For example, the illustrated embodiments of
the optical filter 18 may increase a ratio of the intensity of the
H peak over the intensity of the L peak of the same laser
12.sub.1-12.sub.N by 2 dB or more or by 4 dB or more. Thus, the
optical filter 18 can improve the amplitude optical extinction
ratio by making the corresponding pair of H and L lasing states
closer to optical ON and OFF keying states even if the direct laser
modulation is not too deep.
[0044] In some embodiments, the optical filter 18 is configured
substantially pass each H peak of the N directly modulated lasers
12.sub.1-12.sub.N and to substantially blocks each L peak
thereof.
[0045] Such a configuration may be achieved by if the average
wavelength-channel spacing (AWCS) of the wavelength-channels is
about an integer multiple of the free spectral range (FSR) of the
optical filter 18. For example, AWCS may be equal to FSR, 2FSR,
3FSR, etc., up to a small error. The size of the small error
between the value of the AWCS and the relevant integer multiple of
the FSR limits the maximum size of the array, but the error would
typically, at least, be less than 10 percent of the AWCS. If the
FSR and AWCS satisfy one of the above relations and output spectra
of the N lasers 12.sub.1-12.sub.N have similar shapes up to overall
translations, then, if the optical filter 18 substantially passes
the H peak and strongly relatively attenuates the L peak of the
laser 12.sub.j, the optical filter 18 can substantially pass the H
peaks and strongly attenuate the L peaks of the amplitude and/or
frequency modulated optical carriers output by the other directly
modulated lasers 12.sub.1-12.sub.N.
[0046] Alternatively, the lasers 12.sub.1-12.sub.N may be fixed or
tunable to a non uniform grid for which the spacings between
neighboring grid wavelengths are set to be approximately positive
integer multiples of the FSR of the optical filter 18, e.g., the
spacings may be FSR, 2FSR, 3FSR, etc. For example, spacings between
neighboring grid wavelengths may differ from such positive integer
multiples of the FSR by less than 10 percent of the positive
multiple.
[0047] FIG. 3 illustrates an example of a subsystem 30 in which N
lasers 12.sub.1-12.sub.N are directly modulated to transmit digital
data, in parallel, to a sequence of N corresponding optical
channels, e.g., in the optical transmitter 2 of FIG. 1A. The
subsystem 30 includes an optical multiplexer 16 that combines the
light output by the N lasers 12.sub.1-12.sub.N, a optical sequence
32 that treats and/or aids to monitor the multiplexed output light,
and in some embodiments, a monitoring an adjustment system 52.
[0048] The optical sequence 32 transmits a portion of the
multiplexed output light, i.e., as indicated by a dashed line, from
the optical multiplexer 18 to the transmission optical fiber 4 of
FIG. 1A. In some embodiments, the optical sequence 32 also may
transmit parts of the multiplexed output light to the monitoring
and adjustment system 52.
[0049] The optical sequence 32 includes an optical filter 18 and
may include one or more other bulk or integrated conventional
optical elements, e.g., a tree space sequence of bulk optical
components. The other optical elements may include one or more
optical beam splitters 34, 36; one or more relay lenses 38, 40; one
or more optical isolators 42, 44; and one or more turning mirrors
46, 48, 50. The optical filter 18 may be, e.g., a Fabry-Perot
etalon for which a positive integer multiple of the free spectral
range is about equal to the average wavelength-channel spacing
(AWCS) for the N lasers 12.sub.1-12.sub.N of the array, e.g., up to
.+-.10 percent or less of the AWCS. The filter may be a single or
multi cavity filter, e.g., a multi-cavity optical etalon, which
creates a desired filtering profile. The optional first and second
optical beam splitters 34 and 36 split off first and second parts
of the multiplexed light beam prior to and after filtering by the
optical filter 18, respectively. The optical beam splitters 34 and
36 are not significantly wavelength-dependent over the spectrum of
N lasers 12.sub.1-12.sub.N, e.g., the splitting may be power or
polarization. The optical beam splitters 34 and 36 direct first and
second parts of the optically multiplexed light beam to the
monitoring and adjusting system 52. The relay lenses 38, 40 may aid
to reduce losses due to diffraction and ease the alignment of light
beams through the optical system. The relay lenses 38, 40 may also
provide optimal beam sizes for the optical filter 18 to ensure
correct filter shape and low loss optical losses therein. The relay
lenses 38, 40 are also configured to reduce optical path length
differences for different portions of the optically multiplexed
light beam in the optical filter 18. The optical isolators 42 and
44 reduce optical feedback to the N lasers 12.sub.1 12.sub.N and to
the monitoring and adjusting system 52, respectively. The turning
mirrors 46, 48, 50 may aid to shape or reduce linear dimension(s)
of the footprint of the subsystem 32.
[0050] The monitoring and adjusting system 52 includes first and
second photo-intensity detectors 54 and 56, photo-diodes, and an
electronic controller 58. The first photo-detector 54 makes a
measurement indicative of the intensity of the multiplexed light
beam prior to filtering in the optical filter 18. The second
photo-detector 56 makes a measurement indicative of the intensity
of the multiplexed light beam after filtering in the optical filter
18. The controller 58 is connected by electrical lines to receive
the measurements from the photo-intensity detectors 54, 56 and is
connected by electrical lines to enable adjustment of the output
wavelengths of the N lasers 12.sub.1-12.sub.N of the array based on
the measured light intensities. For example, the electronic
controller 58 may separately control the temperature of the optical
cavities of each of the N lasers 12.sub.1-12.sub.N, e.g., via
currents to individual heaters of the lasers 12.sub.1-12.sub.N (not
shown). Thus, the controller 58 controls and can adjust the output
wavelengths of the N lasers 12.sub.1-12.sub.N.
[0051] In the subsystem 32, each of the N lasers 12.sub.1-12.sub.N
may optionally include a driver 60.sub.1-60.sub.N that is
configured to dither the output frequency of the corresponding
laser 12.sub.1-12.sub.N. For example, the dithering frequencies may
be in the range of about 1 to 10 kilo-Hertz (kHz), about 10-100
kHz, or about 100-1,000 kHz. The j-th driver 60.sub.j may dither
the output frequency of the j-th laser 12, at a frequency that is
different than the frequency that the k-th driver 60.sub.k dithers
the k-th laser 12.sub.k for all k .noteq. j. For that reason, the
contributions of individual ones the N lasers 12.sub.1-12.sub.N to
the multiplexed light beam from the optical multiplexer 16 may be
identified by the corresponding dithering frequencies.
[0052] While the dithering on each laser is described herein as
being performed at a single frequency, alternate embodiments may
use other types of frequency dithering. For example, the frequency
dithering may use spread spectrum techniques in which each lasers
12.sub.1-12.sub.N would be dithered over a band of frequencies,
e.g., as in code-division multiple access (CDMA). In light of the
present disclosure, persons of ordinary skill in the relevant Arts
would readily understand that such alternate forms of frequency
dithering would also provide techniques that enable distinguishing
the contributions of individual ones of the N lasers
12.sub.1-12.sub.N to the multiplexed light beam and to the portion
of the multiplexed beam filtered by the optical filter 18.
[0053] In particular, the electronic controller 58 may accumulate
measurements of the photo-detectors 54, 56 over a sampling period.
The sampling period is typically long compared to the period for
transmitting one data symbol on an wavelength-channel and is
typically short compared to the time over which the output optical
wavelengths of any of the lasers 12.sub.1-12.sub.N are expected to
change significantly. For each sampling period, the electronic
controller 58 can determine a contribution an individual one of the
lasers 12.sub.1-12.sub.N to the intensities measured by the
photo-detectors 54, 56 by evaluating Fourier coefficients of the
measurements at the dithering frequency of the individual one of
the lasers 12.sub.1-12.sub.N. Based on the values of the
contributions, the electronic controller 56 can determine how to
change the output optical wavelength of the individual one of the
lasers 12.sub.1-12.sub.N, e.g., so that the output optical
wavelength of the individual ones of the lasers 12.sub.1-12.sub.N
are approximately wavelength-locked to be at about the AWCS from
the nearest output optical wavelength(s) of other(s) of the lasers
12.sub.1-12.sub.N of the array.
[0054] At a dithering frequency, the ratio of the contribution to
the part of the multiplexed light beam measured in the
photo-detector 56 over the contribution to the part of the
multiplexed light beam, as measured in the photo-detector 54, is
often roughly maximized when a pass band of the optical filter 18
is about centered on 11 peak of the corresponding one of the lasers
12.sub.1-12.sub.N. Such a maximization will typically occur if the
pass band of the optical filter 18 has 3 dB width that is equal to
or less that the 3 dB width of H peak. Then, the optical filter 18
can substantially pass the H peak and simultaneously substantially
attenuate the corresponding L peak when centered at about the
center wavelength of the H peak.
[0055] Alternate embodiments of the subsystem 30 of FIG. 3 may use
alternate ways to distinguish the intensity contributions of the
individual lasers 12.sub.1-12.sub.N in the optically multiplexed
light beam and the filtered portion thereof. For example, the
subsystem 30 may be configured to modulate each individual laser
12.sub.1-12.sub.N with a corresponding and distinct pseudo-random
sequence of digital data. Then, the different pseudo-random
sequences of digital data may be used to identify contributions of
individual ones of the lasers 12.sub.1-12.sub.N. In particular, the
electronic controller 58 may use those parts of the measured light
intensities that temporally correlate with one of the pseudo-random
sequences of digital data as a measure of the contributions of the
one of the lasers 12.sub.1-12.sub.N that transmits that particular
pseudo random sequence. Indeed, any adjustment of an output
wavelength of that one of the lasers may be based on the estimates
of the part(s) of light intensities detected by the photo-detectors
54, 56 that temporally correlate with that particular pseudo-random
sequence.
[0056] FIG. 4 illustrates a method 70 of operating an array of N
lasers to optically transmit digital data, e.g., the lasers
12.sub.1-12.sub.N in FIGS. 1A, 1B, 1C, and/or 3. Here, the integer
N is greater than or equal to one, e.g., N may be 1, 2, 3, 4, 5,
6t, 7, 8, 9, 10 or more.
[0057] The method 70 includes directly modulating a plurality of
lasers such that each laser of the plurality outputs an amplitude
and/or frequency modulated light beam on a corresponding
wavelength-channel (step 72). The wavelength-channels of the
different lasers are spaced apart by an average wavelength-channel
spacing. The various wavelength-channels may or may not be spaced
apart by an approximately wavelength-independent spacing. The
direct modulating generates optical carriers that carry streams of
digital data, e.g., each optical carrier may be modulated by a
different stream of received digital data. For example, the drivers
14.sub.1-14.sub.N of FIGS. 1A-1C and/or 3 drive the lasers
12.sub.1-12.sub.N to output optical carriers that are amplitude
and/or frequency modulated by digital data streams
DATA.sub.1-DATA.sub.N. The amplitude and/or frequency modulation of
the Optical carriers may correspond approximately, in some
embodiments, to optical ON/OFF keying, or may correspond to an
amplitude and/or frequency modulation protocol with more than two
symbol values.
[0058] The method 70 includes optically multiplexing the amplitude
and/or frequency modulated optical carriers that are output by the
lasers to form an optically multiplexed light beam (step 74). The
optical multiplexing may be performed by the N.times.1 optical
multiplexer 16 of FIGS. 1A, 1B, 1C, and/or 3, e.g., which may
receive the amplitude and/or frequency modulated optical carriers
via optical waveguides (OW).
[0059] The method 70 includes optically filtering the optically
multiplexed light beam in an optical filter, e.g., the optical
filter 18 of FIGS. 1A, 1B, 1C, and/or 3 (step 76). The optical
filter has a free spectral range (FSR). The average
wavelength-channel spacing is approximately an integer multiple of
the free spectral range of the optical filter. For example, the
average wavelength-channel spacing may differ from such an integer
multiple of the free spectral range by less than about 10 percent
of the average wavelength-channel spacing. Then, if the optical
filtering of step 76 improves an amplitude optical extinction ratio
for one of the wavelength-channels, the optical filtering can also
improve the amplitude optical extinction ratio for the other(s) of
the N optical channels, i.e., if the spectral intensity peaks are
similarly distributed in each of the wavelength-channels.
[0060] The optical filtering of the step 76 typically more strongly
attenuates one spectral peak each of the directly modulated lasers,
e.g., the L peak of FIG. 2, than the other spectral peak(s), e.g.,
the H peak of FIG. 2, of the same one of the directly modulated
lasers. For example, the low intensity spectral peaks may be
typically more strongly attenuated by 2 dB or more than the
corresponding H peaks or may be more strongly attenuated by 4 dB or
more than the corresponding H peaks. In such embodiments, the
optical filtering of the step 76 improves the amplitude optical
extinction ratio of the amplitude and/or frequency modulated
optical carriers output by the directly modulated lasers.
[0061] FIG. 5 illustrates a method 80 of wavelength-locking an
array of directly laser modulated lasers, e.g., the N lasers
12.sub.1-12.sub.N illustrated in FIGS. 1A, 1B, 1C, and/or 3.
[0062] The method 80 includes optically multiplexing the N
amplitude and/or frequency and/or frequency modulated optical
carriers output by N corresponding lasers during direct laser
modulation of digital data thereby, e.g., using the optical
multiplexer 16 of FIGS. 1A-1C and/or FIG. 3 (step 82). The N
amplitude and/or frequency modulated optical carriers are output,
in parallel, on different wavelength-channels. Thus, the optical
multiplexing produces an optically multiplexed light beam with the
amplitude and/or frequency modulated optical carriers in N
different optical wavelength channels.
[0063] The method 80 may optionally include frequency-dithering the
output optical wavelength of each of the N lasers during the direct
laser modulation thereof. For example, the dithering may be done
with the drivers 60.sub.1-60.sub.N of FIG. 3. The
frequency-dithering the N lasers may be performed in series or in
parallel. If the frequency-dithering is performed in parallel, each
one of the N lasers will typically have a different dithering
frequency than the other(s) of the N lasers. The individual
dithering frequencies may be, e.g., one of the ranges 10 kHz to 10
kHz; 10 kHz to 1,000 kHz; 1,000 kHz to 1 MHz; etc.
[0064] The method 80 optionally includes optically splitting the
multiplexed light beam into a main portion and a first sampling
part, e.g., in optical beam splitter 34 of FIG. 3 (step 84). The
optical splitting directs the main portion towards an optical
filter, e.g., the optical filter 18 of FIGS. 1A-1C and 3, and
directs the sampling part towards an optical intensity detector,
e.g., photo-detector 54 of FIG. 3. The optical splitting is
performed in a substantially wavelength-independent manner so that
intensities of individual wavelength-channels in the first sampling
part are indicative of relative intensities of those
wavelength-channels in the original optically multiplexed light
beam. For example, the optical splitting may involve splitting the
optically multiplexed light beam with a polarization splitter or an
optical power splitter to provide for such substantial
wavelength-independence.
[0065] In embodiments including the optional optical splitting of
step 84, the method 80 also includes electronically measuring
optical intensities of the first sampling part of the optically
multiplexed light beam, i.e., an unfiltered part, at a set of times
in a first temporal averaging period, e.g., in the photo-detector
54 of FIG. 3 (step 86). The first temporal averaging period is
preferably short compared to times over which output wavelengths of
the lasers are expected to substantially change and is typically
long compared to the period for transmitting an optical symbol on
one of the amplitude and/or frequency modulated optical carriers,
e.g., the period may be one to a few milliseconds. The intensities
measured at the sequence of times may be sent, e.g., to the
electronic controller 58 of FIG. 3 for further processing.
[0066] In embodiments including optional optical splitting of step
84, the method 80 also includes electronically estimating relative
contributions of light in individual ones of the
wavelength-channels to the intensities of the first sampling part
measured at the set of times in the first temporal averaging
period, e.g., in the electronic controller 58 of FIG. 3 (step 88).
The estimating step 88 estimates the contributions of individual
ones of the N lasers to the measured intensities and may be
performed in a serial manner or in a parallel manner.
[0067] In a serial embodiment, the estimating of step 88 may
involve sequentially varying the amplitude and/or frequency of the
light output by individual ones of the N lasers or sequentially
frequency-dithering individual ones of the N lasers to determine
the individual contribution of the laser-being-varied to the
measured optical intensities.
[0068] In a parallel embodiment, the estimating step 88 may involve
frequency-dithering the N lasers, in parallel, where each of the
lasers is dithered at a different frequency than the other(s) of
the lasers. In such an embodiment, the dithering frequency can be
used to identify the contribution of individual ones of the N
lasers. Then, the estimating step 90 may involve, e.g., evaluating
Fourier coefficients of Fourier components of the intensities
measured in the first sampling part at the dithering frequencies of
the N lasers. Such Fourier coefficients can be obtained by making
weighted averages over the intensities measured at the set of times
of the first temporal averaging period. Indeed, the Fourier
coefficient of the measured intensities for a particular frequency
is one estimate of the relative contribution of the laser, which
was dithered at that frequency, to the intensities measured for the
first sampling part in the first temporal averaging period. If the
spectral response of the filter at step 84 is known, comparing a
Fourier component before and after filtering by the filter can
enable a determination of the relative position of the wavelength
of the laser emitting said Fourier component.
[0069] The method 80 includes optically filtering the multiplexed
light beam in an optical filter or optically filtering the main
portion of the multiplexed light beam in embodiments where the
optical splitting step 84 is performed (step 90). The optical
filter may be, e.g., optical filter 18 of FIG. 3. The average
wavelength-channel spacing (ACS) of the wavelength-channels may be
about equal to a positive integer multiple of the free spectral
range (FSR) of the optical filter, e.g., up to an error of .+-.10
percent or less of the ACS. For such a free spectral range, the
optical filtering of step 90 can improve amplitude optical
extinction ratios of each of the N amplitude and/or frequency
modulated optical carriers provided that the optical filtering
improves the amplitude optical extinction ratio of one of the
amplitude and/or frequency modulated optical carriers and that the
distribution of light intensity has a similar shape in the
different wavelength-channels. For such a free spectral range, the
optical filtering step 90 can also provide feedback that useable
for wavelength-locking each of the N lasers if the optical
filtering again provides feedback usable to wavelength-lock one of
the N lasers and the distribution of light intensity has a similar
shape in the different wavelength-channels.
[0070] The method 80 includes optically splitting the optically
filtered multiplexed light beam or the optically filtered main
portion thereof into an output portion and a second sampling part,
e.g., in optical beam splitter 56 of FIG. 3 (step 92). The optical
splitting step 92 is also performed in a substantially
wavelength-independent manner so that the contribution of each
wavelength-channel to the intensity of the second sampling part is
indicative of the relative intensity of the individual
wavelength-channel in the optically filtered multiplexed light beam
or the optically filtered main portion thereof. For example, the
optical splitting of step 92 may be performed with a polarization
splitter or an optical power splitter to provide for such
substantial wavelength-independence.
[0071] This optical splitting step 92 typically directs the output
portion of the filtered Multiplexed light beam or the filtered main
portion thereof towards an optical transmission channel. In the
system 10A of FIG. 1A, the output portion may be transmitted
directly or indirectly to the optical transmission channel 4. In
the subsystem 30 of FIG. 3, the output portion is further processed
by the part of the optical sequence 32 formed by the relay lens 40,
mirror 50, and isolator 44 and is then, transmitted to the optical
transmission channel 4.
[0072] This optical splitting step 92 typically directs the second
sampling part to a second optical intensity detector, e.g.,
photo-intensity detector 56 of FIG. 3. Alternatively, rather than
performing the optically splitting step 92 to produce the second
sampling portion, a reflected portion of the multiplexed light beam
from the filter may be used to form the second sampling portion,
directed towards the second optical intensity detector.
[0073] The method 80 includes electronically measuring optical
intensities of the second sampling part of the filtered multiplexed
light beam or the filtered main portion of the multiplexed light
beam at a sequence of times in a second temporal averaging period,
e.g., with the photo-detector 54 of FIG. 3 (step 94). The second
temporal averaging period is again preferably short compared to the
time over which output wavelength(s) of the N lasers arc expected
to substantially change and is typically long compared to a period
for transmitting an optical symbol on one of the amplitude and/or
frequency modulated optical carriers. The second temporal averaging
period may be equal to or about equal to the first temporal
averaging period. The intensities measured at the sequence of times
of the second temporal averaging period may again be sent to the
electronic controller 58 of FIG. 3.
[0074] The method 80 includes electronically estimating relative
contributions of the individual wavelength-channels to the
intensities of the second sampling part measured at the times of
the second temporal averaging period, e.g., in the electronic
controller 58 of FIG. 3 (step 96). The estimating of step 96
involves estimating the contributions of individual ones of the
lasers to the measured intensities and may be performed in a serial
manner or in a parallel-manner. The serial and parallel embodiments
of step 96 may be similar or identical to respective serial and
parallel embodiments described with respect to the estimating step
88 except that the processing is performed on measurements of the
second sampling part during the second temporal averaging period
rather than on measurements of the first sampling part during the
first temporal averaging period. For example, an estimate for the
contribution of a wavelength-channel to the second sampling part
may be taken as the Fourier coefficient for the Fourier component
whose frequency is the dithering frequency of the laser that
outputs an amplitude and/or frequency modulated carrier in that
wavelength-channel.
[0075] The method 80 includes adjusting the output wavelengths of
the individual lasers based on the estimates for the relative
contributions of the individual lasers to the measured intensities
of the second sampling part of the filtered portion of multiplexed
light beam as determined at above step 96 (step 98). The adjusting
step 98 may optionally also be based on the estimates of the step
88 for relative contributions of said individual lasers to the
measured intensities of the first sampling part of the multiplexed
light beam, i.e., the unfiltered part, as determined at above step
88. The adjusting step 98 may be controlled, e.g., by the
controller 58 of FIG. 3, and the output wavelength adjustments may
be implemented by the adjustment of individual heaters (not shown
in FIG. 3) for the N lasers by the controller 58. In some
embodiments, the adjusting step 98 involves adjusting the output
wavelengths of the N lasers to lie on a preselected grid of
wavelength-channels, e.g., a grid in which neighboring channels are
separated by a fixed or wavelength-independent spacing, e.g., about
equal to the average wavelength-channel spacing (AWCS).
[0076] In some embodiments, the estimated intensity contribution of
a laser to the measured intensities of the second sampling part of
the filtered light, as determined at step 96, is used itself to
determine how to vary the output wavelength of that laser. For
example, the output wavelength maybe adjusted to iteratively
maximize the estimated value for this contribution.
[0077] In other embodiments, a ratio of the estimated contribution
of the laser to the measured intensities of the second sampling
part of the filtered light, as determined at step 96, over the
estimated contribution of the same laser to the measured
intensities of the first sampling part of the unfiltered light, as
determined at step 88, is used to determine how to vary the output
wavelength of that laser. For example, the output wavelength of
that laser may be adjusted iteratively to maximize this ratio. As
discussed above, both contributions for this laser may be obtained
as Fourier coefficients for Fourier components of the measured
intensities of the first and second sampling parts at the dithering
frequency of that laser.
[0078] FIGS. 6A-6C illustrate how a laser's contribution to the
measured intensities of the second sampling part during direct
modulation, as estimated at step 96, and the ratio of said
contribution to the same laser's contribution to the measured
intensities of the first sampling part during direct modulation, as
estimated at step 88, can be use to determine the output wavelength
of the laser. In FIGS. 6A-6C, the optical filter performing optical
filtering step 90 has a band pass that is about as wide as the H
spectral peak in the laser's output spectrum. For this optical
filter, the laser's contribution to the intensity of the
multiplexed light beam after the optical filtering is strongly
dependent to the spectral locations of the band passes of the
optical filter and the spectral location of the H and L peaks of
the laser.
[0079] FIGS. 6A, 6B, and 6C indicate configurations in which the
optical filter of the step 90 has a rectangular band pass that is:
[0080] A) centered on the center wavelength of the H peak of the
laser during direct modulation, [0081] B) centered at a wavelength
slight shorter than the center wavelength of the H peak of the
laser during direct modulation, and [0082] C) centered at a
wavelength that is slight longer than the center wavelength of the
H peak of the laser during direct modulation, respectively. The
left portions of FIGS. 6A-6C illustrate the band pass (dotted
lines) of the optical filter of step 90 and the output spectrum of
the laser (solid lines) for these configurations. The right
portions of FIGS. 6A-6C illustrate the wavelength-dependent
intensity of the spectrum after optical filtering at step 90 with
the laser configured as in FIGS. 6A, 6B, and 6C, respectively. The
right portions show that the configuration of FIG. 6A where the
center wavelength of the H peak is approximately centered on the
center wavelength of the optical filter's pass band leads to
filtered light of a larger integrated intensity than the
configurations of FIGS. 6B-6C in which the center wavelength of the
H peak is not centered on the center wavelength of the optical
filter's pass band. For this reason, the wavelength-integrated
intensity of the light beam produced by filtering at step 90 with
an optical filter has maxima when the center wavelengths of the H
peaks of the directly modulated lasers are aligned with the center
wavelengths of pass hands of the optical filter. For that reason,
each laser can be wavelength-locked to output light into a
corresponding band pass of the optical filter used for optical
filtering at the step 90 by adjusting the output wavelength of each
laser to maximize its contribution to the measured intensities of
the second sampling parts as estimated at the step 96.
[0083] In various embodiments, the steps of the method 80 of FIG. 5
may be repeated to approximately wavelength-lock N lasers to a set
of preselected and corresponding output wavelengths.
[0084] In various embodiments, the improvement of amplitude optical
extinction ratios and optical wavelength-locking may be performed
together. For example, the systems 10A-10C and the subsystem 30 of
FIG. 3 may optically filter the modulated optical carriers output
by the lasers 12.sub.1-12.sub.N to both improve amplitude optical
extinction ratios therein and obtain feedback for
wavelength-locking the lasers 12.sub.1-12.sub.N to a preselected
grid of wavelengths. For example, the systems 10A-10C and the
subsystem 30 of FIG. 3 may be configured to perform both the method
70 of FIG. 4 and the method 80 of FIG. 5.
[0085] From the disclosure, drawings, and claims, other embodiments
of the invention will be apparent to those skilled in the art.
* * * * *